The present invention relates to carbon fiber chopped strands and a coating dispersion used in spinning for production of the chopped strands. More particularly, it relates to carbon fiber chopped strands excellent in separating (unbinding) for the purpose of improving dispersibility in a matrix when used as reinforcements for composite materials and a coating dispersion used for production of the strands.
At present, carbon fibers are widely used in various fields. From the viewpoint of starting materials, carbon fibers can be roughly classified into polyacrylonitrile based carbon fibers, rayon based carbon fibers and pitch based carbon fibers. The production cost of the pitch based carbon fibers produced from carbonaceous pitches such as petroleum pitch and coal tar pitch is lower than that of polyacrylonitrile or rayon based carbon fibers because the starting materials for the former are inexpensive as compared with the starting materials for the latter and besides, yield of carbonization is higher in production of pitch based carbon fibers. Thus, pitch based carbon fibers have recently been noticed and many researches have been made thereon.
Carbon fibers are produced, for example, by the following process.
(1) A carbonaceous pitch suitable for production of carbon fibers is prepared from petroleum or coal tar pitches, the resulting carbonaceous pitch is molten by heating and spun into filaments and the resulting filaments are bundled to make a pitch fiber strand.
(2) The pitch fiber strand is infusiblized by heating in an oxidizing gas atmosphere. This step is a step of oxidizing the thermoplastic pitch fibers to convert them into infusiblized fibers which are no longer molten even by heating.
(3) Subsequently, the infusiblized fibers are carbonized or graphitized at a high temperature in an inert atmosphere. By this treatment, volatile component in the infusiblized fibers and thermally unstable portion in the pitch molecules are decomposed and volatilized and the aromatic structure in the molecules is developed into a structure of high carbon content and in some case a structure close to graphite structure. As a result, carbon fibers of high strength and high elasticity are obtained.
This process suffers from the problem that the fibers which constitute the pitch fiber strand fuse together at the time of infusiblization of the bundled pitch fiber strand and various proposals have been made in an attempt to solve this problem. For example, Japanese Patent Kokoku No. Hei 2-2975 has proposed a process for infusiblization by allowing molybdenum disulfide to adhere to the pitch fibers. Furthermore, Japanese Patent Kokai No. Hei 3-33221 has proposed a process for infusiblization by allowing graphite to adhere to the pitch fibers. Japanese Patent Kokoku No. Hei 2-2975 proposed a process which comprises allowing graphite to adhere to the pitch fibers, chopping the pitch fibers and infusiblizing the pitch fibers in the form of chopped strand.
Pitch based carbon fibers are often produced in the form of chopped strands and used in that form. That is, they are produced by bundling carbon fibers and cutting them to short fibers and these are used as reinforcements for matrices such as thermoplastic resins, cements, ceramics and metals by dispersing them in the matrices as filaments of carbon fibers.
The chopped strands must maintain the bundled state before use for easy handling, good transportability and inhibition of fluffing. On the other hand, the chopped strands must be able to be completely and easily dispersed in a matrix as filaments in use.
There are various methods for dispersing the carbon fiber chopped strands in the matrix, but are subject to restriction by the kind of the matrix and molding method of the matrix. The carbon fiber chopped strands produced by the conventional processes have the problem that they can be satisfactorily dispersed by mixers such as an extruder which give strong shearing force, but are difficult to disperse by mixers such as omnimixer which give weak shearing force. Therefore, such carbon fiber chopped strands have been desired which are excellent in dispersibility of carbon fibers, namely, in separating (unbinding) of the strands.